In industrial applications it is often desirable to uniformly roughen organic surfaces on a microscopic level.
Such roughening is especially desirable in the computer industry in the manufacture of first and second level packages for the mounting of information processing devices known as chips.
In such manufacture it is often desirable to attach copper to an organic surface so that the copper does not easily become detached. It is known that metals have improved adhesion to roughened organic surfaces. Various industrial processes which involve uniform roughening of organic surfaces have been developed to improve the attachment of the copper. Most such processes work well but are accompanied by certain problems and disadvantages which are sought to be overcome.
In one process of the prior art a layer of sacrificial copper is laminated onto the organic surface and then stripped away in order to modify the topography of the organic surface so that it will be less smooth and more accepting to the electroless deposition of copper from a plating bath. This process works well and is widely used. However, it has the disadvantage that it consumes a great deal of expensive copper.
In another prior art process which also enjoys wide use, a thin layer of chrome is deposited on an organic surface prior to deposition of copper. It is not completely understood how this process works to improve copper adhesion to the organic surface, but it is believed that the chrome may react chemically or become electrostatically attached to the organic surface. Chrome is an expensive metal, and its use in improving the adhesion of copper to organic materials adds a step to the packaging process. The higher electrical resistivity of chrome may make its use undesirable for many applications. Chrome is normally applied by sputtering or evaporation techniques, which are usually convenient only with structures smaller than printed circuit boards.
Sandblasting is another method used in the prior art to roughen organic surfaces for subsequent deposition of a metal or for subsequent lamination to a second organic material. It is known that roughening the surface of at least one of the layers in an organic laminate structure will improve the adhesion between the roughened surface and the adjacent layer. Sandblasting can be effective for uniformly roughening organic surfaces; however, it is sometimes destructive to the fine features of the surface to be treated and not penetrate deeply into crevices or holes.
It is often necessary in the manufacture of first and second level packages for the computer industry to plate the walls of holes or interconnections, commonly referred to as "through holes" or "vias" which have been drilled or etched in an organic structure, such as a printed circuit board. Drilling smears the organic material from which the board is made and causes it to cover electrical interplanes such as signal, power or ground planes which are desirably left exposed for subsequent connection by the plating of a metal layer on the walls of the through hole. Desmearing of the vias or through holes is necessary. In the past desmearing has been achieved by sandblasting, chemical treatment and plasma treatment, either alone or in various combinations. However it is difficult to achieve effective desmearing with sandblasting, which is especially ineffective in high aspect ratio through holes. Liquid desmearing of the drilled hole walls with materials such as hydrochloric acid and methyl carbitol or by, chromic acid, sulfuric acid and n-methyl pyrolidinone is effective. The use of such chemicals leaves the walls desmeared and sufficiently roughened for subsequent electroless plating of metals. However, these liquids have the disadvantages that they sometimes etch away too much of the organic material, undercutting the interplanes and that they are sometimes undesirable from worker safety and environmental standpoints and presents potential reliability problems with respect to corrosion and migration of the applied metal.
The desmearing of drilled through holes by plasma treatment has been tried and found to be effective. However, plasma desmearing leaves a very smooth surface on the organic material in the walls of the hole, making subsequent electroless deposition of metals very difficult. Roughening of the surface prior to such deposition is necessary.
A paper by R. H. Wheater et al describing a process for roughening the surface of an epoxy resin was published in October of 1983 as a part of the minutes of a May, 1983, plasma symposium of the Electrochemical Society. In that process the epoxy resin was filled with silicon dioxide particles and subjected to an O.sub.2 /CF.sub.4 plasma. The silicon dioxide particles in the resin surface were not affected by the oxygen containing plasma while the epoxy resin therebetween was removed, resulting in a roughening of the surface. Improved adhesion to untreated epoxy and to copper was noted. However, the silicon dioxide particles in the epoxy gave the bulk of the filled epoxy properties of strength, processability and rheology which are undesirable, especially in the manufacture of first level electronic packages such as substrates and semiconductors.